Separations of a gas from admixture with other gases are important industrial processes. In such processes, the objective may be either to obtain a product gas enhanced in a particular gas or a product from which that particular gas as an undesired constituent has been removed. For example, there are commercial scale processes to separate air to obtain nitrogen, oxygen, hydrogen and argon.
Air separation can be accomplished using adsorption processes, in particular, pressure swing (PSA) and vacuum pressure swing types (VPSA). In PSA and VPSA processes, compressed air is pumped through a fixed bed of an adsorbent exhibiting an adsorptive preference for one of the main constituents whereby an effluent product stream enhanced in the non-adsorbed (or lesser adsorbed) constituent is obtained. Compared to cryogenic processes, adsorption processes for air separation require relatively simple equipment and are relatively easy to maintain. Adsorption processes, however, have lower product recovery and are typically less attractive than the cryogenic processes when the goal is to produce large volumes of product with very high purities. For these reasons, improvements in the adsorption processes remain important goals. One principal means of improvement is the discovery and development of better adsorbents.
One way to improve adsorption is to enhance the mass transfer rate of adsorbent materials, particularly those used in PSA and VPSA. With a fast mass transfer rate, one can have short cycle time and low power consumption and therefore high adsorbent productivity and high process efficiency in PSA/VPSA systems and processes. It has been recognized that it is possible to shorten cycle time by reducing particle size of adsorbent aggregates. This recognition has been based upon the assumption that the time needed for adsorbates to travel through the macropores of the agglomerated adsorbent particles limits the adsorption/desorption cycle time, i.e., macropore diffusion is the rate limiting step in adsorption processes.
U.S. Pat. No. 6,425,940 (Chao et al.) describes an improved adsorbent having a specific size-compensated relative rate (SCRR), which measures the effect of the intrinsic properties of the adsorbent (e.g., macropore diameter, macropore shape, macropore volume, macropore distribution) on the adsorption rate, and an air separation process using such adsorbents.
U.S. Pat. No. 4,016,106 (Sawyer et al.) and related U.S. Pat. No. 4,016,107 (Sawyer et al.), U.S. Pat. No. 4,016,108 (Robson), U.S. Pat. No. 4,081,405 (Sawyer), and U.S. Pat. No. 4,081,406 (Sawyer) (collectively, the “Sawyer/Robson patents”) describe a forming process to make catalysts of group VI-B and/or group VIII metals dispersed on alumina supports which are prepared by a solution-based hydrogel method. These patents disclose a method of controlling the pore size distribution and pore volume of the alumina to generate a greater number of pores in size range 100-275 Å by use of the precipitated hydrogel and particularly the use of pore extending polymeric additives. The final pore volume, and pore volume distribution, of the finished alumina is principally determined by the amount of polymer or pore volume extender added to the hydrogel during the pore volume extending step. These patents also disclose a spray drying step to set the pore structure and convert the hydrogel to a boehmite phase. The catalyst composition can then be mulled to an extrudable paste and extruded to form extrudates, and if desired, marumerized to form spheres.
U.S. Pat. No. 6,514,317 (Hirano et al.) describes a beaded zeolite X composition which is useful for separation of hydrogen-based gas mixtures. A marumerizer was used to “dress” the beads which were formed by a blade agitation process. This patent does not teach extrusion for the formation pellets or strands of material which are converted to beads by a marumerizer/spheronizer.
U.S. Pat. No. 6,171,370 (Hirano et al.) describes a beaded zeolite X composition which is useful for air separation made by blade agitation granulation with the beads polished by a marumerizer, but does not disclose how to improve adsorption rate.
U.S. Pat. No. 4,316,819 (Tu et al.) discloses an aluminosilicate composition bound with an organic polymer which is agglomerated by extrusion and improved by spheronizing the shaped products before drying. Although U.S. Pat. No. 4,316,819 discloses the use of extrusion and spheronization to improve the properties of a zeolite (aluminosilicate), it is specifically for use in aqueous sugar separations. There are no teachings regarding making high rate adsorbents for gas separations and/or purifications where silicon dissolution is not an issue. Tu et al. demonstrate the benefits resulting from increasing the density of the adsorbent through spheronization. The teachings from the examples in U.S. Pat. No. 4,316,819 prefer more intense spheronization in regard to both speed and duration, which reflects the different adsorbent properties required for aqueous phase sugar separation than those required in gas phase separations.
Japanese Publication No. 2003055103 (Chisso Corporation) discloses a method for manufacturing agrochemicals wherein the components are mixed, extruded and thereafter converted into spherical particles using a marumerizer. Japanese Publication No. 2003055103 teaches producing particles with high sphericity which can be coated with agents for specific applications (including controlled release of chemicals agents and/or pharmaceutical drugs). Hence, the processing method and conditions are tailored to delivering particles with a highly spherical geometry, where the particles are suitable hosts for the target agents whose release is to be controlled. There is no discussion of high rate.
It would be desirable to make adsorbent beads with higher mass transfer rates compared to adsorbents produced by conventional bead forming processes at a given particle diameter. A method of making high rate adsorbents using an extrusion-spheronization process and more preferably using low pressure extrusion has not been described to date. Typically, adsorbent extrudates have been formed in high pressure extrusion processes and prior art does not indicate the use of low pressure extrusion.
Furthermore, in the past, extrusion-spheronization has been applied almost exclusively to the pharmaceutical and agrochemicals industries, and their agglomeration requirements and performance measures are quite different from those necessary or desirable for air separation processes.